Process and apparatus for separating metals by distillation



Dec. 16, 1969 c. F. BONILLA PROCESS AND APPARATUS FOR SEPARATING METALSBY DISTILLATION Filed 001. 14. 1966 2 Sheets-Shet 1 I Z n Z INVENTOR.C/As /1 Bof/luf:

Arrop/vfv l Ill D. 1s, 1969 c. F. EQNILLA 3,484,233

PROCESSVAND APPARATUS FOR SEPARATING METALS BY DISTILLATION Filed OCT..14. 1966 2 SheBtS`-Sheet 2 s1 i V f f 49 i 1111i i 3; 40-

1 lei 2 f1 111i INVENTOR.

United States Patent Ov U.S. Cl. 7S-63 6 Claims ABSTRACT F THEDISCLOSURE Process and apparatus are disclosed for separating arelatively volatile first metal from a liquid alloy thereof with asecond metal of substantially lesser volatility. This invention involvesmaintaining in a first zone a first pool of a liquid alloy of saidmetals, maintaining in a second zone a second pool of liquid alloy ofsaid metals, the concentration of said first metal being greater in saidsecond pool than in said first pool, evaporating a portion of said firstmetal from said first pool, conducting the thus-formed vapors of saidfirst metal into indirect heat exchange with second pool, condensingsaid vapors by said indirect heat exchange, passing the resultingcondensed liquid phase first metal into said second pool at a pointbeneath the surface of said second pool, evaporating a portion of saidfirst metal from said second pool, conducting said evaporated firstmetal away from said second zone, and maintaining the pressure in saidsecond zone at an absolute level less than the pressure in said firstzone and at a level sufficient to permit the heat transferred by thecondensing vapors from said first Zone to be sufiicient to evaporatesome of the first metal from said second pool.

This invention relates to separation processes and apparatus, and moreparticularly to processes and apparatus for obtaining a relativelyvolatile metal in substantially pure form from an alloy of said metalwith a second metal of substantially lesser volatility.

Mixtures of light metals, such as sodium or potassium, with heavymetals, such as tin or lead, are formed in various industrial processes.One example of such process is the fused salt electrolysis of sodiumchloride using a molten lead electrode as described, for example, inU.S. Patent 3,104,213. The metallic sodium formed is dissolved in themolten lead electrode, forming a liquid alloy containing about to 20% byweight of sodium, with the remainder being lead. It is sometimesdesirable to recover the sodium from this alloy in highly purified formcontaining not more than about one part per million of lead.

The recovery from a sodium-lead alloy of exceptionally high puritysodium is very difiicult to achieve by currently known processes.Sodium-lead mixtures predominating in lead have high boiling points; forexample, a mixture containing sodium and 90% lead by weight has aboiling point of about 900 C. The high operating temperatures necessaryfoi separating sodium and lead favor the use of compact apparatus inorder to minimize heat losses. It is also desirable to use specialmaterials of construction, for example stainless steel or even morerefractory alloys, to resist internal corrosion by the sodium-lead alloyand external oxidation by air at the high operating temperaturesrequired.

Theoretically, sodium and lead could be separated by distillation usingonly a small number of plates, eg. about 2 or 3 plates. Actually it hasbeen found that conventional distillation procses and apparatus, such asthose used in separating petroleum mixtures, will not give pure sodiumcontaining a maximum of one part per million of lead. A considerableamount of entrainment of liquid 3,484,233 Patented Dec. 16, 1969 icedroplets in the vapor phase takes place in the conventional distillationcolumn. The principal reason for this is that vapor from each plate isbuhbled through the liquid on the next higher plate, agitating theliquid and entraining some of the liquid in the form of fine droplets aseach vapor bubble rises to the surface and bursts. Some of thesedroplets settle out on higher plates, but are replaced by other newerones. As a result, liquid droplets may be carried in the vapor stream upthrough the entire distillation column, by-passing a number of platesand passing into the overhead product. In the case of sodium-lead andsimilar alloys, this would result in the presence of relativelysubstantial and undesirable concentrations of lead in the sodium productunless an impractically large number of plates were used.

In addition, when the components differ greatly in volatility, as withsodium lead, vapor bubbling through liquid on a plate approachesequilibrium much less completely than when it is condensed and mixed in,and then revaporized.

The principal object of this invention is to provide a process andapparatus for the recovery of high purity sodium or other relativelyvolatile metal from an alloy of this metal with a second metal ofsubstantially lesser volatility.

According to the process of this invention, a liquid alloy of arelatively volatile first metal and a second metal of substantiallylesser volatility is fractionated in an apparatus having a plurality ofzones, each zone containing a pool of liquid alloy of the two metals. Aportion of the first metal in each zone is evaporated, withdrawn fromsuch Zone, and the vapors are condensed by indirect heat exchange withthe liquid metal in the next zone. The condensate thus produced is thenintroduced into and below the surface of the pool of molten metal in thenext zone. The vapors produced in the last zone consist of highly purefirst metal which are withdrawn as product, except for a small portion,as required, which is condensed and returned to the system as reflux.The Operating pressure maintained in each zone is not only less than theoperating pressure maintained in the preceding zone but is generallyless by an amount greater than that which normally exists indistillation columns in order that the correlativo relationships oftemperature, pressure and alloy composition between the liquid pool in azone and the vapors from the preceding zone permit the total indirectheat exchange condensation of such vapors as aforesaid The totalcondensation of the metal vapor from each zone before it is introducedinto the next zone absolutely transfers any entrainment therein at onceto the liquid phase, preventing its by-passing this next zone in a vaporstream.

Introduction and mixing of the condensed vapor into the pool of metalwell below the surface of the pool minimizes or eliminates the newentrainrnent that `would be generated by the flashing or spontaneous`explosive boiling of said condensed vapor if it emerged near the surfacein the next zone, which zone is operated :at a combination oftemperature and pressure conditions which would otherwise permit suchcondensed vapor to boil.

In order to continue the purification of the sodium or other volatilemetal it is necessary for vapor to continue to rise from each zone tothe next higher one, countercurrent to a descending stream of liquidrefiux, as in a normal distillation column. Accordingly, it is necessaryfor the liquidpool on the above next zone to generate new vapor byvaporization to replace the vapor from the `zone below as it condenses.The heat of vaporization required and employed in generating the newvapor on this next zone is of course that given up by the vapor risingfrom the zone below as it condenses.

An additional desirable feature in applying this invention is that theindirect heat transfer tubes in which the rising vapor condenses be soshaped and placed that the new vapor they generate not be generatedbelow the surface of the pool of molten metal, where it would in turngenerate new entrainment as it bubbles through the surface. Instead, theliquid should be assisted to circulate steadily to the surface of thepool and there to vaporize slowly and uniformly without forming bubbles.This can be accomplished in liquid metals boiling at low temperatures,as primarily contemplated herein, by distributing said heat transfertubes consistently somewhat below the surface of the liquid pool, butwell and uniformly throughout the pool, and by not exceeding areasonable rate of vaporization, such that corresponding to about100,000 B.t.u. of latent heat per hour per square foot of pool surface.

The second metal is recovered from the system as a bottoms product alloycontaining a lower concentration of the first metal than in the feed.

According to a preferred embodiment of this invention, a feed alloy ofsodium and lead containing about to by weight of sodium is fractionatedin two zones to produce substantially pure sodium containing no morethan about one part per million of lead. This highly pure sodium isrecovered as the distillate product. The bottoms product is alead-sodium alloy containing a greater concentration of lead than thefeed alloy.

The apparatus of this invention comprises a plurality of compartmentsarranged in series, each compartment providing a separation zone. Eachcompartment includes means for retaining a pool of liquid alloy and avapor space above the pool. Each compartment maintains itself at itsindividual pressure, the pressure decreasing progressively from thefirst compartment to the last compartment in the series, as in anordinary distillation column. A feed conduit introduces a molten alloyinto one of the compartments in the series and most frequently into thelowest compartment. At least one and generally a plurality of vaporconduits are provided for conducting metal vapor from the vapor space ofeach compartment in the series to the liquid pool of the nextcompartment. These conduits are disposed beneath the surface of liquidin the next compartment, so that the vapors passing therethrough arepassed in indirect heat exchange relationship with the liquid alloy inthe pool, and are thereby condensed before their introduction into theliquid pool. An overflow tube permits return of overflow liquid alloyfrom each pool to the preceding pool in the series. The apparatus alsoincludes an overhead product conduit for recovering the first metal inhighly pure form, and a bottoms product conduit for recovering thesecond metal. A condenser may be provided for condensing a portion ofthe overhead product and returning it to the system.

This invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a vertical elevational view, with parts shown in section, ofthe apparatus according to one embodiment of the invention;

FIG. 2 is a sectional View looking downwardly along line 2--2 of FIG. l;

FIG. 3 is a vertical sectional View of the apparatus according to asecond embodiment of this invention; and

FIG. 4 is a sectional view looking downwardly along line 4-4 of FIG. 3.

Referring now to FIGS. l and 2 and especially to FIG. l, there is showna plural zone fractionation vessel 10 having a tangential feed conduit11 for introduction of a liquid alloy of a relatively Volatile firstmetal and a second metal of substantially lower volatility. The outletend of the feed conduit 11 terminates beneath the surface of a pool 12of liquid alloy in the bottom of vessel 10. This pool is richer in thesecond metal than is the feed alloy entering through feed conduit 11.Vessel 10 has a bottoms product conduit 13 for withdrawing liquid alloyfrom the bottom of pool 12. It is desirable to withdraw the bottoms froma lower point in pool 12, in order to obtain the densest liquid and tominimize stagnant hold-up. Thus, an open-ended sleeve 36 is disposedaround conduit 13. The height of the inlet end of this conduit 13determines the height of molten metal in pool 12. In a typicaloperation, nearly all of the less volatile metal and much of the morevolatile metal is withdrawn as bottoms through conduit 13.

A fluid tight partition 14 divides vessel 10 into a pair of compartments15 and 16, which constitute separation zones. Partition 14 includes agenerally circular plate 17 which conforms in cross section to that ofthe vessel 10 except for a sector adjacent the inner wall of vessel 10which is cut out, a vertical plate 18 which is `welded along its loweredge to plate 17 and along its sides to the inner wall of vessel 10, anda small horizontal plate 19 which conforms in cross-sectional shape tothe cutout portion of plate 17 and which is welded to vertical plate 18and to the inner wall of vessel 10. Plates 18 and 19 together with theinner wall of vessel 10 form a chimney 20 in the first compartment 15,and plates 17 and 18 together with vessel wall 10 Iform a well 21 forcontaining a pool 22 of molten alloy in second compartment 16. This pool22 contains both the first and second metals, but is richer in the first(i.e., the more volatile) metal than either pool 12 or the alloy feedmixture entering through pipe 11.

An overflow pipe 24 permits return of alloy from second compartment 16to first compartment 15. The inlet of pipe 24 is sufficiently aboveplate 14 so that pool 22 of molten alloy has an appreciable depth. Theoutlet end of pipe 24 is located beneath the surface of molten alloypool 12, so that pipe 24 cannot constitute a by-pass through which vaporcan ow from compartment 15 to compartment 16, avoiding condensationtubes 25, hereafter to be described.

This invention provides a novel structure for conveying metal vapor fromthe fir-st compartment 15 to the second compartment 16. This structureincludes a plurality of serpentine condensation tubes 25 having theirinlet ends 26 in chimney 20 of first compartment 15. Tubes 25 aredisposed beneath the surface of molten alloy in the pool 22 in thesecond compartment 16. The tubes 25 as shown are S-shaped, comprisingthree straight lengths of tubing joined by U bends. The outlet ends 27of tubes 25 are down-turned and are the lowest points of the tubes.

Vapor of the more volatile metal, with only small amounts of the lessvolatile metal, enters the inlet ends 26 of tubes 25, and is totallycondensed during its passage through tubes 25 by indirect heat exchangewith the alloy in pool 22. The only portion of the gases and vapors intubes 25 which is not condensed is non-condensable gas, such asnitrogen, when present, which will be compressed into the exit ends oftubes 25 by the incoming vapor, to eventually exit through outlet 27 ingreatly reduced volume compared to the total vapor stream. An amount ofthe rst metal in pool 22 is evaporated by the heat transferred oncondensation. The amount of liquid evaporated from pool 22 is comparableto the amount of vapor condensed in tubes 25. However, it is evaporateduniformly enough from the surface of pool 22 so that undesirablyvigorous agitation is not caused thereby, and so that vaporization isfrom the surface rather than by bubble formation or boiling below thesurface. Excessive agitation and boiling are avoided to prevententrainment of liquid droplets, which contain appreciable quantities ofthe second metal, in the vapor of highly pure first metal which isformed in `compartment 16. The entire heat requirements for the secondcompartment 16 may be furnished by the vapor condensed in tubes 2S.

Liquid from pool 22 backs up only a short distance into the outlet ends27 of tubes 25, until the gas pressure inside tubes 25 and the statichead of the liquid in pool 22 adjacent the outlet ends 27 are equalized.The pressure of vapor in tubes 25 is not allowed to exceed the statichead of liquid at outlet ends 27, because this would result in vaporbubbles passing through liquid alloy pool 22 with resultant entrainmentof liquid droplets in the vapor above pool 22.

Condensation of the rst metal in tubes 25 and simultaneous evaporationof the first metal from alloy pool 22 is facilitated by the fact thatthe pressure of vapor in tubes 25 is higher than the pressure existingin zone 16.

Non-condensable gases can be removed from pipe 25 by means of awithdrawal `conduit 28 which is connected to a vacuum pump (not shown).Tubes 25 are connected near their outlet ends to a horizontal manifold29, so that a single vacuum pipe 28 will rernOve non-condensable gasesfrom the entire bank of tubes 25. A condenser 34 surrounds withdrawalconduit 28 on the exterior of vessel 10. This condenser condenses anymetal vapors which may be present in the gas stream flowing through withdrawal conduit 28. The resulting condensate travels down Wardly throughwithdrawal conduit 28 and is discharged through the outlet end 27 of oneor more of the tubes 25. Withdrawal conduit 28 also includes a manuallyoperated Valve 31 for cutting the Vcommunication with the vacuum pump,to simplify control when the vacuum pump is not needed during operation.

Vapor formed in the second compartment 16 is removed from vessel throughoverhead product outlet 32. The vapor leaving the apparatus throughoutlet conduit 32 consists of the first metal in highly purified form,with only traces of the less volatile metal. For example, where sodiumand lead are the two metals being separated, the vapor in outlet 32 issubstantially pure sodium which may contain no more than about one partper million of lead. A reflux condenser 33 surrounds outlet conduit 32above vessel 10. This reflux condenser condenses a portion of the vaporsin outlet conduit 32. These vapors descend along the inner wall ofconduit 32. and are collected in an annular trap 34, which is in theshape of a channel. A downcomer tube 35 returns condensate from trap 34to the molten alloy pool 22. The outlet end of downcomer pipe 35 islocated beneath the surface of metal in pool 22 so as to prevent hashingor explosive boiling of this metal when it comes into contact with thehotter metal of pool 22.

Referring now to FIGS. 3 and 4, a modified form of the apparatus of thisinvention is shown. This apparatus includes a vessel 40, which isdivided into a first compartment 41 and a second compartment 42 by meansof a horizontal plate 43, which is welded to the inner walls of vessel40 in uid-tight engagement therewiLh. As in the embodiment of FIGS. land 2, the first compartment 41 is maintained at a higher pressure thanthe second cornpartment 42. The first and second compartments 41 and 42contain pools 44 and 45 respectively of molten alloy` A tangential alloyfeed conduit 46 supplies molten alloy to the rst compartment 41 beneaththe surface of alloy pool 44 therein. The alloy in pool 44 contains ahigher concentration of the second metal than does the alloy feedentering through feed conduit 46. In some cases, the alloy in pool 44maybe virtually pure second metal. The alloy in pool 45 is richer in thefirst metal than either the feed alloy or the alloy in pool 44. In thisembodiment of the invention, as in the embodiment of FIGS. 1 and 2, theterm first compartment is used to refer to the compartment which isoperated at the highest pressure and which contains the liquid poolhaving the higest percentage of the less volatile metal. However, inthis embodiment the first compartment is located above the sccondcompartment, opposite to the embodiment of FIGS. 1 and 2. A

Vapors formed in the first-compartment 41, which consist essentially ofthe rst metal with only small amounts of the second metal, are conveyedfrom the vapor space of compartment 41 downwardly through tubes 47.These tubes 47 have their inlet ends in the vapor space of compartment41. Their outlet ends are substantially below the surface of the alloyin pool 45. This affords an opportunity for condensation of all of themetal vapors in tubes 47 tbefore they are introduced into pool 45.

An annular plate 48, joined to the eXteriors of tubes 47 by welding,positions tubes 47 accurately and provides structural reinforcement forthe tubes.

Liquid from pool 45 rises a short distance into tubes 47, the distancebeing determined by the pressure in tubes 47 and in compartment 41. The:pressure in compartment 41 is greater than the pressure in compartment42, so as to afford an opportunity for indirect heat exchange betweenvapors in tubes 47 with the liquid in pool 45. This indirect heatexchange results in total condensation of the vapors in tubes 47, andvaporization of an equivalent portion of the rst metal in alloy pool 45.The rate of vaporization taking place as a result of such condensationis well distributed over pool 45, and is insufficient to cause undueagitation of the liquid in pool 45, and boiling below the surface ofpool 45, so as to avoid entrainment of liquid from this pool in thevapors rising from it. At the same time, the pressure in compartment 41and tubes 47 must not exceed the pressure of liquid in pool 45 at theoutlet ends of tubes 47, for this would result in gas bubbles emergingfrom tubes 47 and traveling upwardly through outlet pool 45, entrainingliquid from this pool as the vapor bubbles break the liquid surface incompartment 42. However, in this embodiment any non-condensable gasentering compartment 42 with the vapor from compartment 41 would only beable to leave compartment 42 by being compressed and concentrated in thebottom of tubes 47, and thus would produce a small amount of entrainmentthereby.

An outlet conduit 49 is provided for removal of a small stream of moltenalloy from pool 45 in second compartment 42. In the first embodiment,and in most distillation columns this would be designated reflux, andwould liow to the rst compartment. In. this embodiment, however, aspecial pump would be required. And since in this application the amountof this stream is only a few percent of the main Streams, it ispreferably added directly to the bottoms product leaving the first`compartment through tube 51, and thereby returns to the manufacturingprocess. The height of the inlet end of return conduit 49 determines theheight of liquid alloy in pool 45.

The drain line 50 is provided at the bottom of vessel 40 for removingliquid from compartment 42 during shutdown as desired. This drain line5t) is kept closed during normal operation.

The bottoms product is withdrawn from liquid pool 44 in firstcompartment 41 through a bottoms product outlet conduit 51, which islocated near the top of vessel 40. The location of the bottoms productoutlet conduit 51 determines the height of liquid alloy in pool 44. Theterm bottoms product is used herein to denote the liquid productcontaining a preponderant proportion of the less volatile component, inconformity to the terminology generally used in distillation. It will benoted that the compartment 41 containing the liquid alloy of leastvolatility is at the top of the apparatus, rather than on the bottom.Bottoms product outlet 51 is placed above feed conduit 46, so that theoutlet end of feed conduit 46 is submerged beneath the surface of pool44.

Vapors of highly pure rst metal are withdrawn from the secondcompartment 42 through vapor outlet conduit 52. A reflux condenser 53surrounds vapor product conduit 52 above vessel 4G. This causescondensation of a portion of the vapors in conduit 52. This `condensateis collected in an annular trap 54 in tube 52 and returned to liquidpool 45 through return tube .55, the outlet end of which is beneath thesurface of pool 45 so as to avoid agitation and flash or explosiveboiling when it comes in contact With the hotter liquid of pool 45.

This invention will now be described further with reference to aspecific embodiment thereof. For the sake of illustration, the processwill be applied to FIG. 1, although this process can be Carried outequally well in either of the illustrated embodiments of the inventionwith minor modifications.

A molten alloy feed containing 90% by weight of lead and 10% by weightof sodium is introduced into vessel 10 through pipe 11. The flow ratewill be taken as 100 parts by weight per hour, consisting of 90 parts byWeight of lead and 10 parts `by weight of sodium. This alloy feed entersthe first compartment beneath the surface of alloy pool 12 therein. Thealloy pool 12 has a composition of 9.78 percent by weight of sodium and90.22 percent by weight of lead, and a temperature of 800 C. Evaporationtakes place from the surface of this pool, and the pressure in the vaporspace above this pool is 59 mm. of mercury.

The bottoms stream issuing through conduit 13 has the same compositionand temperature as alloy pool 12, and has a flow rate of 99.76 parts byweight per hour consisting of 90 parts by weight per hour of lead and9.76 parts by weight per hour of sodium.

The vapor stream rising from alloy pool 12 contains approximately 0.24percent by weight of lead, and consists of 0.25 part by weight per hourof sodium and 0.0006 part by weight per` hour of lead. This vapor passesinto tubes 25, and is condensed therein at about 50 mm. of mercury at640 C. The condensate is introduced into alloy pool 22 at a depthsubstantially below the surface thereof.

Alloy pool 22 is at a temperature of 610 C. Evaporation takes place fromthe surface of this pool, generating sodium vapor at a pressure of 31mm. of mercury and at a flow rate of 0.25 part by weight per hour. Thevapor, which is highly pure sodium, is removed through vapor outletconduit 32. The coolant flow rate through reflux condenser 33 isadjusted so that the vapor withdrawn from the system has a purity of99.9999 percent by weight of sodium. The reflux rate is quite small,typically amounting to about 0.01 part by weight per hour, consistingalmost entirely of sodium. In some instances it may be necessary toincrease the reiiux rate slightly in order to compensate for minorentrainment or nonequilibrium in the generation of the final sodiumvapor. The alloy pool 22 contains approximately 6 percent by weight oflead and 94 percent by weight of sodium. The composition of this pool isinliuenced by the reflux rate. The liquid reflux stream descendingthrough tube 24 has the same composition as the alloy pool 22. Theliquid rate through tube 24 is about 0.0106 part by weight per hour,comprising 0.01 part by weight of sodium and 0.0006 part by weight oflead.

In the event that the vapors from first compartment 15 contain anynon-condensable gases, these are removed through withdrawal conduit 28.

Various modifications can be made by those skilled in the art. Forexample, more than two separation Zones may be used, although bothembodiments illustrated show apparatus having two zones. It is seldomnecessary to use more than two separation zones, because of the greatdifference in volatilities of the two metals, and :because the absenceof agitation of liquid in either pool 12 or pool 22 (or in pools 44 and45) prevents entrainment of liquid droplets in the vapor obtained fromany separation zone. This is an important factor in obtaining highpurity sodium or other like metal. Any number of separation zones may beused, and in each case the first zone has the greatest operatingpressure and the alloy pool of lowest volatility, and each succeedingzone in the series is operated at a lower pressure and has an alloy poolricher in the first metal and consequently of greater volatility. Allseparation zones are connected in series, so that the vapor from eachseparation zone (except the last) passes to the next zone and the liquidfrom each separation zone (except the first) passes to the precedingzone. A bottoms richer than the alloy feed in the second metal iswithdrawn from the first zone, and highly pure first metal is withdrawnfrom the last zone as a vapor.

For the sake of convenience and heat economy, all separation zones arepreferably housed in a single vessel as is illustrated in bothembodiments of the invention described herein. However, separate vesselshousing the different separation zones may be used.

It is necessary to provide thermal insulation around all outer walls ofthe apparatus, to minimize heat losses and the ensuing operationaldifficulties. The provision of electrical resistances for other heatersembedded in the thermal insulation is desirable, for heating up theapparatus after a shutdown and for prevention of heat losses from theliquid metals and vapors. A Sheet metal double wall may be providedbelow the walls 14, 17 and 18, if desired, in order to minimize thecondensation of vapors arising from alloy pool 12. The space betweensuch double wall may be vented to the vapor space of the secondcompartment 15, or may be drained into alloy pool 12 by means of asubmerged pipe, in order to prevent filling of this space with liquidmetal.

An external heat source, such as an electric, fuel fired, or otherheater can be provided if desired, either outside vessel 10 or beneaththe surface of liquid pool 12 inside the vessel, in order to provideheat.

The withdrawal conduit 28 for removal of non-condensable gases, and theassociated manifold 29 and condenser 30, may be omitted where there isno non-condensable gas or air leaks in the system.

While this invention has been described primarily with respect to theobtaining of pure sodi-um from mixtures of sodium and lead, it will beunderstood that other light and relatively volatile metals, such aspotassium, lithium and magnesium, may be substituted for sodium as thefirst metal, and that other heavy metals, notably tin, which are ofsubstantially lesser volatility than the first metal, may be substitutedfor lead as the second metal. Although the second metal is ofappreciably lower volatility than the first metal, the second metalnevertheless is preferably not a metal having a very high boiling point,or difficulty may be encountered in finding suitable materials ofconstruction.

The process may be operated over a wide range of pressures, providedthat the absolute pressure in each zone is less than the pressure in thepreceding zone by an amount sufficient to permit condensation of thevapors of the more volatile metal from one zone by indirect heatexchange with the alloy pool of the next-following zone. Throughout thesystem, the pressure may range from above atmospheric to less thanatmospheric, such as down to l mm. of mercury. Advantageously, theentire system is operated at sub-atmospheric pressures to permit the useof lower temperatures than if operated at atmospheric pressure.

While this invention has been described with reference to specificembodiments thereof, it shall be understood that these are merely :byway of illustration and not by way of limitation of the scope of thisinvention.

What is claimed is:

1. A process for separating a relatively volatile first metal from aliquid alloy thereof with a second metal of substantially lesservolatility, which process comprises maintaining in a first zone a rstpool of a liquid alloy of said metals, maintaining in a second zone asecond pool of liquid alloy of said metals, the concentration of saidfirst metal being greater in said second pool than in said first pool,evaporating a portion of said first metal from said first pool,conducting the thus-formed vapors of said first metal into indirect heatexchange with said second pool, completely condenscing said vapors ofsaid first metal by said indirect heat exchange, passing the resultingcondensed liquid phase rst metal into said second pool at a pointbeneath the surface of said second pool, evaporating a portion of saidfirst metal from said second pool, conducting said evaporated firstmetal away from said second zone, and maintaining the pressure in saidsecond zone at an absolute level less than the pressure in said firstzone and at a level sufficient to permit the heat transferred by thecondensing vapors from said first zone to be sufiicient to evaporatesome of the first metal from said second pool.

2. A process for separating a relatively volatile first metal from aliquid alloy thereof with a second metal of substantially lesservolatility, which process comprises maintaining in a first zone a firstpool of a liquid alloy of said metals, maintaining in a second zone asecond pool of liquid alloy of said metals, the concentration of saidfirst metal being greater in said second pool than in said first pool,evaporating a portion of said first metal from said first pool,conducting the thus-formed vapors of said first metal into indirect heatexchange with said second pool, condensing said vapors by said indirectheat exchange, separating non-condensable gases from said condensedliquid phase first metal and withdrawing the non-condensable gases undervacuum, passing the resulting condensed liquid phase first metal intosaid second pool at a point beneath the surface of said second pool,evaporating a portion of said first metal away from said second zone,and maintaining the pressure in said second zone at an absolute levelless than the pressure in said first zone and at a level sufficient topermit the heat transferred by the condensing vapors from said firstZone to be sufiicient to evaporate some of the first metal from saidsecond pool.

3. A process according to claim 2 in which said first metal is sodiumand said second metal is lead.

4. Apparatus for recovering a relatively volatile first metal from aliquid alloy thereof with a second metal of substantially lesservolatility, said apparatus comprising a plurality of compartmentsarranged in series and providing a plurality of separation zones, eachcompartment including means for retaining a pool of liquid alloy and avapor space above said pool, means for maintaining said compartments atseparate predetermined pressures, said pressures decreasingprogressively from the first compartment to the last compartment in theseries, a feed conduit for introducing a liquid alloy of said first andsecond metals into said apparatus, a bottom conduit for withdrawing aliquid alloy richer in the second metal than the feed alloy, an overheadconduit for withdrawing a vapor of substantially pure first metal fromthe last compartment, at least one condensation tu-be extending from thevapor space of each compartment preceding the last compartment forconducting vapors therefrom, said tube being positioned beneath theintended liquid alloy surface level in said next compartment and adaptedfor indirect heat exchange relationship with said alloy and terminatingin an outlet opening beneath said liquid alloy surface level, wherebyvapors of said first metal flowing from said vapor space are condensedprior to introduction into the liquid alloy in said next compartment, awithdrawal conduit for non-condensable gases in communication with saidcondensation tube, and means for placing said withdrawal conduit incommunication with a source of high vacuum.

5. Apparatus according to claim 4 including means for condensing atleast a portion of said substantially pure first metal vapor in saidoverhead conduit and means for returning at least a portion of theresulting condensate.

6. Apparatus according to claim 4 including overflow conduit means forconveying liquid alloy from each compartment to the precedingcompartment.

References Cited UNITED STATES PATENTS 165,201 7/1875 Beam et al202--158 870,747 11/ 1907 Planckh 202--237 X 1,418,885 6/1922 Schulze202-158 X 1,748,855 2/1930 Teter 202-158 X 1,854,002 4/1932 Subkow202158 X 1,967,718 7/1934 Moeller 202--158 2,312,811 3/1943 Gentil75--67 X 2,416,255 2/ 1947 Griswold et al 75-67 X 2,685,505 8/1954Deyrup 75-66 L. WEWAYNE RUTLEDGE, Primary Examiner H. W. TARRING II,Assistant Examiner U.S. Cl. X.R.

